Langmuir 1994,10, 1801-1806
1801
Methanol Chemisorption and Decomposition on FeAl( 110): A High-Resolution Electron Energy Loss Spectroscopy and Temperature Programmed Desorption Study Bor-Ru Sheu and D. R. Strongin' Department of Chemistry, State University of New York a t Stony Brook, Stony Brook, New York 11794 Received October 29,1993. I n Final Form: March 11, 1994' The adsorption and decompositionof methanolon FeAl(110)have been investigated with high-resolution electron energy loss spectroscopy (HREELS) and temperature programmed desorption (TPD). The adsorption of methanol on FeAl(110) at 110 K is shown to be primarily dissociative, resulting in the formation of a methoxy overlayer. HREELS shows that the methoxy species are stable, at least up to 400 K. Heating to higher temperatures results in methoxy decomposition, with both C-H and C-0 bond breaking. TPD data suggest that in addition to gaseous hydrogen and methane evolution, the desorption of methyl radicals occurs near 540 K. A characteristic spectrum of aluminumoxide is obtainedby annealing the methoxy-covered FeAl(110) surface to lo00 K. 1. Introduction A number of high-resolution electron energy loss spectroscopy (HREELS) studies have investigated the chemisorption of methanol on various metals, including Fe(100),13 Fe(110),4 A1(110),5Al(lll),6t7 Ni(llO),S1l Ni(111),12 Cu(100),13J4 c~(llO),'~ Pt(111),'6 Pt(lW),l7Pd(111),18 Pd(110),I9 Pd( 100),2ORh( 111),21Rh( 100),22Ru(OOl),=and M0(100).~All these investigations have shown that methanol undergoes 0-H bond scission in a specific temperature range to form methoxy and that the thermal stability of this intermediate depends on the substrate. For example, methoxy decomposition on Pt(ll1) occurs near 150K,16whiledecomposition of methoxyon Fe(100),3 Fe(110),4 and A1(111)6 does not occur until near 450 K. In general, studies of the transition metals show that CO and H2 (and CH2O for O/Cu) are the primary products that
* To whom correspondence should be addressed: Tel(516)6329043;FAX (516)632-7960. 0 Abstract published in Advance ACS Abstracts, May 1, 1994. (1)Albert, M. R.; Lu, J.-P.; Bemasek, S. L.;Dwyer, D. J. Surf. Sci. 1989,221,197. (2) Lu,J.-P.; Albert, M.; Bernasek, S. L.; Dwyer, D. J. Surf. Sci. 1989, 218,1. (3)Lu,J.-P.; Albert, M. R.; Bernasek, S. L.Surf. Sei. 1991,258,269. (4)McBreen, P. H.; Erley, W.; Ibach, H. Surf. Sci. 1983,133,L469. (5)Waddill, G.D.; Kesmodel, L. L. Surf. Sci. Lett. 1987,182,L248. (6)Chen, J. G.;Basu, P.; Ng, L.;Yates, J. T., Jr. Surf. Sei. 1988,194, 397. .. (7)Basu, P.; Chen, J. G.; Ng, L.;Colaianni, M. L.;Yates, J. T.,Jr. J. Chem. Phys. 1988,89,2406. (8)Richter, L. J.; Ho, W. J. Chem. Phys. 1985,83,2569. (9)Richter, L. J.; Ho, W.J. Vac. Sci. Technol. 1985,A3, 1549. (10)Richter. L.J.: Gumev. _ .B. A.: Villarrubia. J. S.:. Ho.. W. Chem. ~ h y s . i e t t1964, . iii, 185. (11)Bare, S. R.;Stroecio, J. A.; Ho,W. Surf. Sci. 1985,150,399. (12)Demuth, J. E.; Ibach, H. Chem. Phys. Lett. 1979,60,395. (13)Andersson, S.;Pereson, M. Phys. Rev. B 1981,24,3659. (14)Sexton, B. A. Surf. Sci. 1979,88,299. (15)Sexton, B. A.; Hughes, A. E.; Avery, N. R. Surf. Sci. 1985,155,366. (16)Sexton, B. A. Surf. Sci. 1981,102,271. (17)Kizhakevariam N.; Stuve, E. M. Surf. Sci. 1993,286,246. (18)Gates, J. A.; Kesmodel, L. L. J. Catal. 1983,83,437. (19)Bhattacharya, A. K.; Cheaters, M. A.; Pemble, M. E.; Sheppard, N. Surf. Sei. 1988,206,L845. (20)Chrbtmann, K.;Demuth, J. E. J. Chem. Phy8. 1982,76, 6308, 6318. (21)Houtman, C.; Bartaau, M. A. Langmuir 1990,6,1558. (22)Parmeter, J. E.; Jiang, X.; Goodman, D. W. Surf. Sci. 1990,240, 85. (23)Hrbek, J.; dePaola, R. A.; Hoffman, F. M. J. Chem. Phys. 1984, 81,2818. (24)Miles, S. L.;Bernasek, S. L.; Gland, J. L. J. Phys. Chem. 1983, 87,1626.
result from methoxy decomposition. In contrast, methoxy decomposition on A1 results in methane production. It has also been shown that methoxy decomposition on oxygen-modified Mo results in methyl desorption.25 In the present research, we investigate the chemisorption of methanol on the FeAl(110) alloy (crystallizes in a CsCl structure) surface with HREELS and temperature programmed desorption (TPD). Previous research has investigated the adsorption and thermal decomposition of methanol on FeA1(110),26with ultraviolet and X-ray photoelectron spectroscopy (UPS and XPS). This prior study suggests that partial decomposition of methanol on the aluminide surfaces results in the formation of methoxy, CH30(a),which interacts strongly with the A1 component. Results presented in this paper show more conclusively by vibrational spectroscopy that methoxy is a stable intermediate on this aluminide surface. HREELS data also suggest that methanol undergoes 0-H bond scission on FeAl(110) at 110 K to form methoxy. Furthermore, methoxy is shown to be stable on FeAl(110) until 400 K and that substantial decomposition occurs by 500 K. Finally, TPD results show that the decompositionreactions of methanol on FeAl(110)result in the desorption of methyl radicals, in addition to methane and hydrogen.
2. Experimental Techniques and Sample Preparation Experiments were performed in a two-level stainless steel ultrahigh-vacuum (UHV) system with a base pressure of 2 X 10-loTorr (ion and turbomolecular pumped). The top level of the chamber is equipped with a quadrupole mass spectrometer, low energy electron diffraction (LEED) optics, double pass cylindricalmirror analyzer,X-ray source, differentially-pumped ultraviolet source, and an ion gun. The lower level houses a HREEL spectrometer (MacAllister Technical Services). The composition and preparation of the FeAl(110) crystal (diameter,1cm; thickness,0.2 cm) used in this research has been describedin prior publications.2efl Carbon and oxygen contamination on the sample were detected by Auger electron spectroscopy (AES)and XPS, when initially introduced into the UHV chamber. Cleaning of FeAI(110)was accomplished by repeated cycles of 500-eV argon ion sputter and anneal (1073K for 10min, 1173K for 2 min) cycles. The brief anneal at 1173 K was necessary (25)Serafin, J. G.; Friend, C. M. J.Am. Chem. SOC.1989,111,8967. (26)Sheu, B.-R.; Strongin, D. R. J. Phys. Chem. 1993,97,10144. (27)Gleason, N.R.; Strongin, D. R. Surf. Sci. 1993,295,306.
0743-7463/94/2410-1801$04.50/00 1994 American Chemical Society
1802 Langmuir, Vol. 10, No. 6, 1994
Sheu and Strongin
to dissolve oxygen,which persisted after sputtering,into the FeAl bulk. A reproducible LEED pattern was obtained for FeAl(110) after this cleaning procedure. Recent research suggests that the surface structure of the FeAl(110)crystal is highly dependent on the preparation conditions%and that under our preparation conditions the clean surface will be slightly enriched in A1 and incommensurate with the bulk. The details of this surface structure, however, still need to be determined. The FeAl(110) sample was mounted on a liquid nitrogen cryostat with cooling capabilities down to 110 K. Three Ta support wires (0.025cm diameter)were spot welded to each side of the sample. The ends of the support wires were spot welded to Ta tabs that were mechanically fastened to a liquid nitrogen cryostat. Heating of the sample was accomplished by passing current through the Ta support wires. The heatingrate for TPD experiments was 7 f 1 K/s. The ionization region of the mass spectrometer, which was used in the TPD experiments, was housed in an enclosure with an aperture of 6 mm diameter.The sample was placed 0.3 cm away from the aperture during TPD experiments. Chromel-alumel (type K) thermocouple wire was spot welded to the bottom of the samplefor accuratetemperature measurements. Methanol (CHaOH, HPLC grade, Fisherchemical; CHBOD, >99.5 atom % D, Sigma Chemical Co.; and CD30D,>99.8 atom 7% D, CambridgeIsotope Laboratories) was purified by numerous freeze-pump-thaw cycles. The methanol exposures quoted in this paper are in langmuirs (1langmuir = 10-6Torr s), and they are corrected for the cracking efficiencies of molecules in the ion gauge used for pressure measurements. Dosingof FeAl(110)with methanol for TPD and HREELS experiments was accomplished by placing the sample about 0.5 cm away from a 0.32 cm diameter tube, through which methanol was admitted into the chamber. The effective pressure at the sample was much higher than the pressure at the ionization gauge during these doses. Exposures quoted in this paper for TPD experiments have been corrected for this enhancement factor, which was determined in separate experiments. HREELS spectra presented in this paper were acquired at a specular detectionangle (ei = 6d = 60"). A primary beam energy of 3.5 eV was used in all the HREELS experiments. A typical counting rate of lo4Hz and a resolution of 96 cm-l (fwhm)were measured for the elastically scattered electrons from clean FeAl(110). The resolution and counting rate of the primary beam varied from 100 to 120 cm-l and from 5 X lo2 to 5 X 103 Hz, respectively, after exposure of FeAl(110)to methanol. There is an uncertainty of A10 cm-l in all the positions of the EELS features shown in this paper.
3.0 Results and Interpretation 3.1. Multilayer Adsorption of Methanol. HREELS spectra of FeAl(110) after exposure to 10 langmuirs of CH30H, 10 langmuirs of CH30D, and 12 langmuirs of CD3OD are shown in Figure 1. The CH30H spectrum shows six EELS features, at 735, 1050, 1155, 1471,2955, and 3223 cm-l. Comparison of the CH3OH and CH30D spectrum shows that the 3223 and 735 cm-' features in the CH30H spectrum shift to 2474 and 538 cm-l, respectively, when the hydroxyl hydrogen is replaced by deuterium. Comparison of the CD30D and CH30H spectra shows that the 2955-cm-' feature in the CH30H spectrum shifts and separates into two resolvable peaks at 2195 and 2063 cm-l in the CD30D spectrum. Furthermore, the 1471-cm-1 mode of CH3OH shifts to 1103 cm-' for CD30D. Based on these isotope-induced frequency shifts and after comparison to vibrational frequencies reported for solid methanol29 and condensed methanol on metal surfaces,1s6J1J5,24 the EELS features in the CH30H spectrum can be readily assigned. The 2955-cm-' mode is assigned to C-H stretching [v(CH3)I,the 1471-cm-1 feature to the CH3 deformation [6(CH3)1mode, and the 3223- and 735~~~
~
~
(28) Graupner, H.; Zehner, D. M., private communication. (29) Falk,M.; Whalley, E.J. Chem. Phys. 1961, 34, 1554.
,
I
1
2195 2440
x
\ 10341155 I I 1471
125
CD,OD 12 L
2963
CH30D
10 L 2955
0
500
1000 1500 2000 2 5 0 0 3 0 0 0 3 5 0 0 4000 4500
-1
) Figure 1. HREELS spectra of FeAl(110) after exposure to 10 E n e r g y Loss ( c m
langmuirs of CH30H,10 langmuirs of CH30D,and 12 langmuirs of CD30D. cm-l modes to 0-H stretching [v(OH)I and bending modes [6(OH)] of CHBOH, respectively. Finally, the EELS features a t 1050 and 1155 cm-' in the CH30H spectrum are assigned to C-0 [v(CO)l and CH3 rocking [r(CH3)1 modes, respectively. Note that the CD3 rocking [r(CD,)I feature in the CD30D spectrum is presumably convoluted with the C-0 stretching feature a t 985 cm-l. The energy loss positions and assignments for CHsOH, CH30D, and CD30D are compiled in Table 1. 3.2 Methoxy Formation. Figure 2 displays HREELS spectra after FeAl(110) is exposed to 0.4, 1.2, and 10 langmuirs of methanol at 110 K. The top spectrum of Figure 2 is obtained by exposing FeAl(110)to 10langmuirs of methanol and then heating the sample to the desorption temperature of methanol multilayers ( - 150 K). The top two spectra of Figure 2 show that after heating to 150 K, the loss features, assigned to the 0-H stretching (3223cm-l) and bending (735 cm-') modes are eliminated, but EELS features attributed to C-H stretching, CH3 deformation, CH3 rocking, and C-0 stretching modes remain unchanged in both energy position and intensity. On the basis of these observations we postulate that CH30(a) species are responsible for the 150 K spectrum. Furthermore, an EELS feature appears near 560 cm-l, upon heating to 150 K, and we assign this feature to the metal-0 stretching mode [v(M-O)] of methoxy. All these assignments are consistent with previous research that has spectroscopically identified methoxy on a variety of metal surfaces.'-24 The bottom two HREELS spectra of Figure 2 are similar to the 150 K spectrum in that 0-H stretching and bending modes are absent, and EELS features attributed to C-H stretching, CH3 deformation, CH3 rocking, and C-0 stretching modes are present. Furthermore, modes at 540 and 560 cm-l appear in the 0.4- and 1.2-langmuir spectra, respectively, and we again assign these features to the v(M-0) mode of m e t h ~ x y .We ~ ~infer from these data, therefore, that some fraction of methanol undergoes 0-H bond scission upon adsorption on FeAl(110) at 110 K. We
Methanol Chemisorption on FeAl
Langmuir, Vol. 10, No. 6, 1994 1803
Table 1. Summary of the Vibrational Frequencies (cm-1) for Condensed Methanol and Adsorbed Methoxy on FeAl(l10) 10 langmuirs 10 langmuirs 12 langmuirs 0.4 langmuirs 1.2 langmuirs CHsO(a) CDsO(a) modes CH3OH CHaOD CDaOD CHSO(a)110 K CHsO(a)110 K 150 K 250 K 2474 2963
2440 2955
2955
2955
2195
2200
2063 1471
2063 1471
1471
1471
1155
1155
1155
1010
1040
1050
985
540
560
560
540
1103 1155
I
I
,
I
I
1034
985
538
568
8
I
I
1103
I
%,d=110 K
CH30H/FeA1( 110)
r125
().
CH,OH/FeAl( 110) 1.
I
125
\
lo50
1155
1471
500 K
2955 3223
400 K r125 735
kl
I
150
CD30D/FeA1( 110)
\
fi
g85
1125
540
I I
500
(b)
2195 2Oag I /
0
K
q440 I
1000 1500 2000 2500 3000 3500 4000 4500
Energy Loss (cm-') Figure 2. HREELS spectra of FeAl(110)after variousexposures of methanol at 110 K. The 10-langmuir spectrum exhibits characteristic features of a CH3OH multilayer. Exposing FeA1(110) to 0.4,and 1.2 langmuirs of CHsOH leads to the formation of methoxy. The top spectrum is for a methoxy overlayer prepared by heating 10-langmuir CHsOH dosed FeAl(110) to 150 K.
do not rule out the possibility, however, that some undissociated methanol coexists with methoxy a t 110 K.31 Loss features resulting from the surface hydrogen, which results from 0-H bond cleavage, are not resolved in these spectra. The absence of these features is probably due to the weak dynamical dipole moment of surface hydrogen and the possible overlap of atomic hydrogen features with energy loss features of surface meth0xy.3~933We note that (30) We mention that the v(C-0) mode intensity in these spectra is similar to the intensities of the CHa-relatedmodes (similarrelative mode intensities between C-O and CHs modes are observed for methoxy on Al(111) as shown in ref 6). In contrast, the C-O mode of methoxy on transition metals is typically far more intense than CHa modes (e&, see ref 11). Our spectra of methoxy on FeAl(110) might suggest an inclined methoxy structure,but off-specularHREELS measurementsare needed to support this possibility. (31) A low scattering cross section cannot be ruled out as a reason for the absence of 0-H stretching (-3220 cm-1) and bending (-735 cm-9 modes in the 0.4-and 1.2-langmuirsspectra. (32) Erley, W.; Baro, A. M. Surf. Sci. Lett. 1981, 112, L759. (33) Erskine, J. L.; Strong, R. L. Phys. Rev. B 1982,%, 5547.
0
500
1000 1500 2000 2500 3000 3500 4000 4500
-1
Energy Loss (cm ) Figure 3. HREELS spectra showing thermal stabilities of (a) CHsO(a)and (b) CDsO(a) on FeAl(110). Spectra are obtained by heating in a stepwisefashion. Note that the bottom spectrum is for a CDSODmultilayer. CDsO(a)is formed by heating to 250 K. The spectra show that methoxy is stable until 400 K, and substantial decomposition occurs after heating to 500 K. All spectra are obtained at -130 K. a feature at 1950cm-l appears in the 0.4-and 1.2-langmuir spectra. We attribute this feature to carbon monoxide contamination, since this moleculeexhibits a v(C-0) mode near 1950 cm-l on pure Fe.34 3.3. Thermal Decomposition of the Methoxy Overlayer. The bottom spectrum of Figure 3a shows a HREELS spectrum of a methoxy overlayer prepared by heating a methanol multilayer on FeAl(110)to 150 K. The remaining spectra in the this figure are obtained by heating CH30/FeA1(110) to the indicated temperatures and then cooling to 110 K. Heating the methoxy overlayer to 230 K increases the intensity of the broad feature centered at 540 cm-1, which is assigned to the v(M-0) stretching (34)Seip, U.;Tsai, M.-C.;Christmann, K.; KClppers, J.; Ertl, G. Surf. Sci. 1984, 139,29.
Sheu and Strongin
1804 Langmuir, Vol. IO, No. 6, 1994 mode.35 Other energy loss features of methoxy are unchanged at this temperature. Further heating of CH3OI FeAl(110) to 400 K causes a slight shift of the dM-0) mode to 560 cm-1, but the other energy loss features show no significant changes. Based on the similarity between the 230 and 400 K spectrum we argue that the methoxy overlayer is stable, at least, up to 400 K. We also mention that desorption of surface hydrogen, which results from the loss of the hydroxyl hydrogen from methanol, occurs near 300 K.26 The mode near 560 cm-', which persists from 130 to 400 K, is not affected by the desorption of this hydrogen, supporting the assignment of the 540-560 cm-l HREELS feature to the v(M-0) mode of methoxy, rather than to a v(M-0) mode of associatively adsorbed methanol. Further support for the existence and thermal stability of methoxy is obtained by examining HREELS spectra for the CD30D/FeA1(110) system, shown in Figure 3b. Exposure of FeAl(110) to 1 2 langmuirs of CD3OD results in a condensed layer, consistent with the presence of 0-D stretching (2440 cm-1) and the 0-D bending (568 cm-l) features. Heating this surface to 250 K desorbs the methanol-& multilayers and uncovers the adsorbed layer. Loss features attributable to 0-D stretching and bending are absent in the 250 K spectrum, while all the features attributed to C-0 and CD3 modes remain (see Table 1). These data suggest that CD30(a) exists on FeAl(110) a t 250 K. Furthermore, the 250 K spectrum shows a feature a t 540 cm-l, which we assign to the v(M-0) mode. Note that both CDsO(a) and CHSO(a)spectra exhibit a feature near 540 cm-l, indicating that the position of this feature is relatively insensitive to isotopic substitution of the methyl group, consistent with this mode being assigned to the metal-0 stretch. The HREELS spectrum obtained after heating methoxy to 400 K is similar to the 250 K spectrum, again indicating that methoxy is stable on FeAl(110), at least up to 400 K. The top spectrum of Figure 3a shows that decomposition of methoxy monolayer occurs after heating CH30IFeAl(110) to 500 K. The v(C-0) mode is markedly reduced in the 500 K spectrum and the intensities of the C-H stretching, CH3 rocking, and CH3 deformation modes are reduced approximately to two-fifth of the corresponding features in the 400 K spectrum. Also, the 500 K spectrum exhibits new EELS features in the region between 500 and 1000 cm-1 that we attribute to oxygen andlor carbon fragment-metal vibrations. The conclusion from HREELS, that methoxy on FeAl(110) undergoes substantial decomposition between 400 and 500 K, is consistent with prior XPS and UPS measurements.26 Heating CH30H/FeA1(110) to higher temperatures results in the formation of A1203. We show this oxide formation with HREELS data exhibited in Figure 4, which is obtained by exposing FeAl(110) to 4 langmuirs of CH3OH at 110 K, and then heating to 1000 K. The spectrum shows relatively intense EELS features a t 422, 642, and 878 cm-l and less intense loss features at 1298, 1510, and 1740 cm-'. A small C-H stretching feature a t 2952 cm-' is also present and we attribute this to partially hydrogenated fragments (CH,) on FeAl(110). This hydrocarbon feature may result from the readsorption of methanol after heating to 1000 K and then cooling to 130 K, where data (35) It is not clear why the mode at 540 cm-1 sharpens after heating. The possibility that the change in this mode is due to further 0-H bond cleavage of previously undissociated methanol and formation of methoxy cannot be supported by these data, since features due to 0-H stretching and bending are absent a t 150 K. A change in methoxy orientation (or ordering) is another possible explanation. In this circumstance, however, one would expect the C-0 and C-H modes to show a change in relative intensity, since these modes also are sensitive to the geometry of surfacebound methoxy.
,
,
I
I
I
A120 3/FeA1( 110)
a-ta ~
h
(1
x 50
x 250
2852
0
500
1000 1500 2000 2 5 0 0 3 0 0 0 3 5 0 0 4000 4500
E n e r g y Loss ( c m
-1
)
Figure 4. HREELS spectrum showing the characteristicEELS loss features of aluminum oxide that result from heating CH3OH dosed FeAl(110) to 1000 K.
are acquired. The energy loss position and relative intensities of the EELS features between 400 and 1750 cm-' are in excellent agreement with features present in spectra of A1203 supported on Al(lll),3a0 Mo(110),4l NiA1(111),42 and NiAl(1 3.4. Gaseous Products. Parts a and b of Figure 5 exhibit TPD data for FeAl(110) after exposure to 1.2 langmuirs of CH30H and CD30D, respectively. The mle = 2 (Hz+) and 31 (e.g., CH30+)signals are attributed to the desorption of hydrogen and methanol from FeA1(1lo), respectively. A methanol exposure of 1.2 langmuirs presumably produces a near saturation coverage of methoxyat 110KonFeAl(110), since highermethanolexposures do not lead to an increase in the hydrogen yield. The only significant change that occurs as the exposure is increased past 1.2 langmuirs is that a sharp peak near 150 K appears (i.e., a t m / e = 31) during TPD, which is attributed to methanol desorption from a condensed layer (this peak grows indefinitely with increasing exposure). Both mle = 16 and 15 signals, corresponding to CH4+ and CH3+,respectively, are detected during CHBOHTPD. An analysis of these data shows that the mle 16:15 peak area ratio (for peaks near 540 K) is 0.85. This ratio is less than the mle 16:15 intensity ratio of 1.2, which we have obtained for standard CH4 samples at identical mass spectrometer settings. An interpretation of these data can be arrived at after examining the CDBOD-TPD data (36) Strong, R. L.; Firey, B.; de Wette, F. W.; Erskin, J. L. Ph.ys. Reu. B 1982,26, 3483. (37) Strong, R. L.; Firey, B.; de Wette, F. W.; Erskin, J. L. J.Electron Spectrosc. Relat. Phenom. 1983,29, 187. (38) Strong, R. L.; Erskine, J. L. J. VQC.Sci. TechnoE. 1985, A3, 243. (39) Chen, J. G.: Crowell. J. E.; Yates, J. T.. Jr. Phvs. Rev. B 1987,35, 5299. (40) Chen, J. G.; Crowell, J. E. Yates, J. T., Jr. Phys. Rev. B 1986,33, 1436. (41) Chen, P. J.; Colaianni, M. L.; Yates, J. T., Jr. Phys. Reu. B 1990, 41, 8025.
(42) Franchy, R.; Wuttig, W.; Ibach, H. Surf. Sci. 1987,189/190,438. (43) Jaeger, R. M.; Kuhlenbeck, H.;Freund, H.-J.; Wuttig, M.; Hoffmann, W.; Franchy, R.; Ibach, H. Surf. Sci. 1991, 259, 235.
Langmuir, Vol. 10, No. 6, 1994 1805
Methanol Chemisorption on FeAl (a)
1.2L CH30H/FeA1( 110)
A
-
J
m/e=31 I
(b)
I
I I
I I
and that a significant portion of the mle 16 intensity exhibited in Figure 5a is due to a wall reaction between CH3 and surface hydrogen. We note that in a prior p u b l i ~ a t i o nof ~ ~ours * ~ the ~ desorption of gaseous methyl from FeAl(110) was overlooked, due to this facile CH3 H CH4 wall reaction. On the basis of these methanold4 results, the yield of methyl from the FeAl(110) surface is estimated to be 3.0 times greater than that of methane product. This estimate is arrived a t by first subtracting the CD4 and CDJH daughter ion contribution to the mle = 18 signal. The sum of the integrated mle = 19 and deconvoluted mle = 18 peak areas is then divided by the integrated m / e = 20 peak area to determine the relative yield of CD3 and CD4. We view this estimate as a lower limit for the relative yield. For example, the possibility that some CD3 is converted to nonvolatile products via wall reactions is not accounted for, and the cross section for ionization of CD4 has been taken to be equal to that of CD3. Analogous TPD data have been recorded after a 0.5-langmuir CD30D exposure, and in this circumstance the yield of CD3 is estimated to be a factor of 4.0 greater than the CD4 yield. While the reason for this change in relative yield is not clear, we attribute the decrease in methane desorption at lower exposure to the decreased availability of surface hydrogen.
I I
I I
I I
I I
1.2L CD,OD/FeAl(llO)
h
+
4.0. Discussion ,
,
I
I
I
I
Temperature ( K )
Figure 5. TPD spectra of (a) 1.2 langmuirs CHsOH/FeA1(110) and (b) 1.2 langmuirs CDsOD/FeAI(110). Hs (m/e= 2) and CH3OH (m/e = 31 corresponds to cracking fragment of CH30H) desorb from FeAl(110), after exposure to 1.2 langmuirs of CH3OH. Data presented for 1.2 langmuirs CDaOD/FeAl(llO) show that both CDs and CDd desorb during TPD. CDsH+ (m/e = 19) is attributed to a wall reaction between CD3 and hydrogen.
exhibited in Figure 5b. We attribute the peak intensity near 540 K in the m / e = 20 spectrum to CD4+, resulting from ionization of CD4 product. The mle = 18 signal intensity at the same temperature is due to CD3+,but its relative intensity is too large to be due solelyto the cracking of CD4. These data, therefore, suggest that CD3, in addition to CD4, desorbs from FeAl(110) during CD30D TPD, with a peak maximum near 540 K.44 Presumably, the peak in the mle = 19 (CDsH+)spectrum centered near 540 K is due to the presence of CD3H. The presence of this species is probably due to the reaction of CD3 with surface hydrogen on the walls within the vacuum chamber (CD3 + H CDaH), as has been pointed out in previous r e s e a r ~ h . 2 5 * It ~ ~is7noted that peaks below 200 K in these spectra are thought to be due, at least in part, to the desorption of CD30D from FeA1(110).@ In view of these methanold( results, we conclude that the desorption of methyl is the dominant reaction channel
-
(44) No products,which yield a m/e value greater than 20, are detected in this temperature range. (45) Creighton, J. R. Surf. Sci. 1990,234, 287. (46) Lin, J.-L.; Bent, B. E. J. Am. Chem. SOC.1993, 115, 2849. (47) Lin, J.-L.; Bent, B. E. J. Phys. Chem. 1993,97,9713. (48) The sharp onset to the TPD peaks for CDBOD,below 200 K is believed to be due to the desorption from Ta support wires attached to the FeAl(110) crystal (the same effect is observed in some of the CH3OH/FeAl(110) TPD peaks). These support wires heat up much more rapidly than the crystal. At least some of the m/e = 18 intensity can be attributed to the cracking of CDsOD ( m / e = 36 signal due to the parent ion CD30D+ is also detected). We feel, however, that the m/e = 19 and 20 intensity below 200 K is due mostly to reactions on the Ta supports. We do not feel this affects any of our conclusions arrived at from analysis of the desorption peaks near 540 K.
Results presented in this paper suggest that methanol undergoes 0-H bond scission at 110 K on FeAl(110) and that this intermediate decomposes to a significant extent between 400 and 500 K. We have also shown results that suggest that both methyl radicals and methane desorb from FeAl(110) near 540 K. In this section, we briefly discuss some of our results in view of prior research of the monometallic Fe and A1 surfaces. It is emphasized that any comparison is only of a qualitative nature, since FeAl has a unique electronic (due to hybridization of A1 and Fe electronic levels) and geometric structure, which is not available to the corresponding monometallic surfaces. 4.1. Methoxy Adsorption. Prior research suggests that methoxy primarily interacts with the A1 component of FeAl(110). This contention is based on the similarity of the electronic structure of methoxy on different aluminide surfaces (i.e., FeAl(110)andpolycrystallineTiAl and NiA1).26 It is interesting then to compare the energy loss position of the u(M-0) mode for methoxy on FeA1(110), which might be expected to be sensitive to the adsorbing element, to the corresponding frequencies for methoxy on separate Fe and A1 surfaces. HREELS of methoxy on FeAl(110) shows a metal-0 stretching frequency that varies between 540 and 560 cm-l depending on the methoxy coverage. Methoxy on Fe(100) exhibits a v(Fe-0) frequency between 330 and 405 cm-l,' while the corresponding u(A1-0) mode on Al(111) varies from 630 to 655 cm-lS6JIt is clear that the elemental composition of the methoxy adsorption site on FeAl(110) cannot be ascertained by comparison of the u(M-0) mode to the corresponding mode on monometallic A1 and Fe. On the basis of our previous results, however, we still favor a surface picture that has methoxy primarily interacting with the A1 component of FeA1(110),but it is evident that the strong hybridization of the electronic states of A1 with those of Fe precludes such a simple comparison. It is (49) Reference 26 reports the desorption of CHI from polycrystalline NiAland TiAl at peaktemperaturesof 590 and 450K, respectively.Recent results in our laboratory suggest that methyl groups also desorb at these temperatures from NiAl and TiAl, and this will be discussed in future publications.
1806 Langmuir, Vol. 10, No. 6, 1994
mentioned that previous calculations suggest that the A1 component of FeAl actually has an increased electron density, with respect to pure Al,50but further calculations are needed to investigate the consequences of this electronic structure for the bonding of methoxy. 42. Thermal Decomposition of Methoxy and Methyl Radical Desorption. Methoxy is relatively stable on FeAl(110) until temperatures near 400 K and substantial decomposition occurs by 500 K. HREELS shows that the C-0 stretching mode of methoxy is greatly reduced by 500 K, while the intensity of hydrocarbon modes remain relatively intense. We infer from these spectroscopic data that some methoxy is present, but a significant portion has decomposed and has resulted in the formation of adsorbed CH, species. Previous research has also suggested that aluminum oxide is present a t these temperatures.26 The overlap of features below 700 cm-' in the 500 K HREELS spectrum makes it difficult to definitively assign any features to metal-carbon vibrations of carbonaceous species, since the formation of aluminum oxide produces relatively intense features in this region. Some information about the reactions of the carbonaceous fragments can be inferred from our TPD results that show that dehydrogenation of these species on FeAl(110)results in hydrogen desorption between 400 and 700 K. Methoxy decomposition on Fe( 100) leads to hydrogen desorption in a similar temperature range (between 400 and 500 K).51 In contrast, previous research shows that no hydrogen is evolved in the temperature range of 250-700 K when methoxy decomposes on A1(111).6 A simple comparison of these TPD results suggests that electronic states of the alloy, presumably those with strong contributions from the transition metal, are intimately involved in the dehydrogenation reactions of the hydrocarbon fragments. In addition to hydrogen, decomposition of methoxy results in the production of methyl radicals and methane. We note here that methane production is a product that desorbs from monometallic A16 during methanol decomposition, consistent with methoxy interacting strongly with (50) Schultz, P. A.; Davenport, J. W. Scr. Metall. 1992, 27, 629. (51) Benziger, J. B.;Madix, R. J. J. Catal. 1980, 65, 36.
Sheu and Strongin
the A1 component of FeAl(110). Serafin and Friend have shown that methoxy decomposition on oxygen-modified Mo(ll0) leads to methyl desorption near 590 K.25 Based on TPD and XPS results, it was postulated that cleavage of the C-0 bond of methoxy resulted in the direct ejection of methyl radicals into the gas phase. Subsequent calculations by Shiller and Anderson52support this methyl ejection picture. The calculations indicate that the C-0 bond in methoxy adsorbed on Mo has a bond strength of 1.4 eV (compared to 3.7 eV for methanol), making the direct ejection of methyl radicals from surface methoxide an energetically favorable desorption channel near 590 K. Our TPD data show that methyl radicals are evolved at a similar temperature (-540 K) from a oxygen-covered FeAl(110) surface, suggesting that the same type of desorption mechanism may be operative on FeAl(110). In our circumstance, however, TPD results suggest that the formation of methane is a competing process with methyl desorption on FeAl(110). The similarity of the peak desorption temperatures of CDBand CD4 suggests that the desorption kinetics of both these products are controlled by a common reaction step; presumably cleavage of the C-0 bond. 5. Summary The adsorption and thermal decomposition of methanol on FeAl(110) have been investigated by HREELS and TPD. The adsorption of methanol on FeAl(110) at 110 K is shown to be primarily dissociative, resulting in the formation of a methoxy overlayer. Methoxy species are found to be stable up to a temperature of 400 K. A t higher temperatures, methyl radicals, methane, and hydrogen desorb, and partially hydrogenated carbon and oxygen decomposition fragments result. Heating to 1000K results in the formation of aluminum oxide.
Acknowledgment. Support of this research by the National Science Foundation through a NSF Young Investigator Award (NYI) is greatly appreciated (Grant DMR-9258544). We also appreciate General Electric for supplying a single crystal ingot of FeA1. (52) Shiller, P.; Anderson, A. B. J. Phys. Chem. 1991, 95, 1396.
